I saw that too. I have identified the video FX as the cause. These are scanned at startup during "Creating Windows...". That takes some time. There are now more plug-ins on my machine. With the Video-FX window closed, MSP15 opens quicker, but not as fast as MSP12.
matthias-krutz, thank you, this solve just a little bit. It was 30 seconds. Now spend 21 second. Vegas Pro 15 open in less than 10 seconds. This is a BUG that Magix need to solve.